The invention generally relates to check valves used in pumping operations. More specifically, the invention relates to a check valve with a profiled entrance for reducing net positive suction head for piston and plunger pumps.
Check valves are devices that allow fluid to flow through a passageway in one direction but block flow in the reverse direction. The force of gravity and/or the action of a spring aids in closing the valve. FIG. 1 shows an example of a conventional check valve assembly. As shown therein, the major components of a check valve include: a valve body 16, a spring retainer 17, a valve 18, and biasing member 12 in compression between the valve and the spring retainer.
Check valves are used in a variety of applications, from regulating flow in HPLC machines to downhole drilling operations. Because check valves are used universally, in many types of media, they are prone to damage, including stuck or missing discs, backstop tapping, seat tapping, disc flutter, disc stud pin wear, hinge pin wear, and flow leakage. One of the major problems occurring with check valves without sufficient suction head(pressure), is cavitation.
Cavitation is the process in which a liquid changes to a vapor due to a reduction in pressure below liquid vapor pressure. Currently, almost all check valves for piston and plunger pumps have sharp corners at valve entrances or have a very small chamfer or radius, just enough to break the sharp corner. The result of this configuration is vena contracta. Vena contracta is defined as the contracted portion of a liquid jet at and near the orifice from which it issues. The fluid stream 50 shown in FIG. 2 contracting through a minimum diameter 51, is the prime mover for cavitation at check valve inlets. The sharp edges 52 in the entrance 53 cause flow separation, which results in non-recoverable pressure loss. Basically, the sudden increase in the velocity of the pumped liquid as fluid passes from a large flow area to a smaller flow area reduces the inlet pressure, sometimes below the liquid vapor pressure, resulting in the formation of gas and bubbles. The bubbles are caught up and swept upward along the inside cavity. Somewhere along the cavity, the pressure may once again drop below the vapor pressure and cause the bubbles to collapse. Implosions of these vapor pockets can be so rapid that a rumbling/cracking noise is produced. The hydraulic impacts of the shock waves caused by the collapsing bubbles are strong enough to cause minute areas of fatigue on the metal piston or plunger surfaces. Depending on the severity of the cavitation, a decrease in pump performance may also be noted. Cavitation damage to the pump may range from minor pitting to catastrophic failure and depends on the pumped fluid characteristics, energy levels, and duration of cavitation.
Thus, if the suction head of a given pump, namely the energy per lb. (due to pressure, velocity or elevation) required by a liquid to remain a fluid, cannot be raised above the vaporization line by decreasing the temperature or increasing the pressure, cavitation will occur. Cavitation often occurs on pumps in offshore platforms due to space constraints; there is not room available for equipment to house large flow regions, which would allow for minimal pressure reduction, thereby reducing the risk of cavitation. Instead, the equipment promotes small flow regions with many pressure drops, leading to frequent cavitation and premature damage of fluid end components.
The first reaction to a cavitation problem is typically to check the net positive suction head available (NPSHa), measured at the suction flange, and compare it to the net positive suction head required (NPSHr). The NPSHa is a characteristic of the system and is defined as the energy which is in a liquid at the suction connection of the pump over and above that energy in the liquid due to its vapor pressure. The NPSHr is a characteristic of the pump design. It is determined by test or computation and is the energy needed to fill a pump on the suction side and overcome the friction and pressure losses from the suction connection to that point in the pump at which more energy is added; the NPSHr is the minimum head required to prevent cavitation with a given liquid at a given flowrate. The ratio of NPSHa/NPSHr must be sufficiently large to prevent formation of cavitation bubbles.
Normally, the NPSHr plotted on the traditional pump curve is based on a 3% head loss due to cavitation, a convention established many years ago in the Hydraulic Institute of Standards. Permitting a head loss this large means that at some higher flow condition cavitation would already have begun before performance loss was noticed.
For this reason, it is imperative that a margin be provided between the NPSHr and the NPSHa at the desired operating conditions. Further, the NPSHr will actually tend to increase with a reduction in flow.
A reasonable margin of 8 ft of water at rated flow rate is commonly accepted by end users for most services. For known problem applications, such as vacuum tower bottoms and some solvents, this margin is often increased to 10 ft.
The present invention is a check valve that includes a profiled entrance for reducing net positive suction head required. Profiled is defined as being shaped into a particular, predetermined form to streamline flow and minimize vena contracta. The profiled entrance offers an improvement over traditional sharp-cornered entrances by allowing the nozzle to require a lower pressure at the same flow rate. By requiring a lower inlet pressure, the total pressure loss in the pump is reduced, which in turn, reduces the net positive suction head required.